Pancreatic cancer is the fourth most common cause of cancer death in the world, and it has a poor prognosis. For all stages combined, the 1- and 5-year relative survival rates are 25% and 6%, respectively; the median survival for locally advanced and for metastatic disease, which collectively represent over 80% of individuals, is about 10 and 6 months respectively. It is estimated that in the United States in 2012 there will be 43,920 new cases and 37,390 deaths.
Hedgehog (Hh) and Sonic Hedgehog (Shh) are signaling proteins that mediate growth and patterning during embryonic development. These proteins act as morphogens to form long and short range signaling gradients. Hh is expressed in flies, while vertebrates express 3 family members: Sonic, Indian and Desert, of which Shh is the best studied. Shh regulates limb development, cell proliferation and differentiation. In adult tissues, aberrant Shh expression or signaling is implicated in the biogenesis of multiple human cancers, including medulloblastoma, basal cell carcinoma, liver, pancreatic and urogenital tumors [See Pasca di Magliano, M., and Hebrok, M. (2003) Hedgehog signalling in cancer formation and maintenance, Nat Rev Cancer 3, 903-911.]
Hedgehog proteins undergo a unique set of post-translational processing reactions. Shh is synthesized as a 45 kDa precursor that traffics through the secretory pathway. After the signal sequence is removed, Shh undergoes autocleavage to generate a 19 kDa N-terminal signaling molecule, ShhN. During this reaction, cholesterol is attached to the C-terminus of ShhN. In addition, the N-terminal cysteine residue of ShhN is modified by palmitoylation. Unlike nearly all other known palmitoylated proteins, palmitate is attached via an amide bond to the N-terminus of ShhN. Palmitoylation of Hh and Shh is critical for effective long- and short-range signaling. Mutation of the N-terminal Cys to Ser or Ala results in a mutant protein with little or no activity in vivo or in vitro. Attachment of palmitate to Shh is catalyzed by the multipass membrane protein Hhat (Hedgehog acyltransferase). Hhat is a member of the membrane-bound O-acyl transferase (MBOAT) family. Most MBOAT family members catalyze transfer of long chain fatty acids to hydroxyl groups of lipids; however, Hhat is one of three MBOAT proteins that transfer fatty acids to protein substrates. In each case, fatty acid modification of the substrate protein is essential for its signaling function.
The normal adult pancreas does not express Shh. However, aberrant Shh expression can occur in the mature pancreas, where it plays a critical role in promoting pancreatic cancer [See Morton, J. P., and Lewis, B. C. (2007) “Shh signaling and pancreatic cancer: implications for therapy?”, Cell Cycle 6, 1553-1557.] Aberrant expression of Shh drives proliferation of pancreatic cancer cells and formation of pancreatic intraepithelial neoplasms, and Hedgehog signaling is one of the core pathways altered in all human pancreatic cancers. Mouse models of pancreatic cancer reveal that Shh functions synergistically with activated K-Ras to promote and maintain tumorigenesis, while inhibition of Shh signaling blocks pancreatic cancer invasion and metastasis [See Olive et al. (2009) “Inhibition of Hedgehog signaling enhances delivery of chemotherapy in a mouse model of pancreatic cancer”, Science 324, 1457-1461 and Feldmann et al. (2007) “Blockade of hedgehog signaling inhibits pancreatic cancer invasion and metastases: a new paradigm for combination therapy in solid cancers”, Cancer Res. 67, 2187-2196.]
There is an urgent need for novel therapeutics to treat pancreatic cancer. We describe herein Hhat inhibitors that block Shh palmitoylation, and thus provide opportunities for efficacious treatment of pancreatic cancer.